The Open Agent Architecture (OAA), developed and used for several years at SRI International, makes it possible for software services to be provided through the cooperative efforts of distributed collections of autonomous agents. Communication and cooperation between agents are brokered by one or more facilitators, which are responsible for matching requests, from users and agents, with descriptions of the capabilities of other agents. Thus it is not generally required that a user or agent know the identities, locations, or number of other agents involved in satisfying a request. OAA is structured so as to minimize the effort involved in creating new agents and "wrapping" legacy applications, written in various languages and operating on various platforms; to encourage the reuse of existing agents; and to allow for dynamism and flexibility in the makeup ofagent communities. Distinguishing features of OAA as compared with related work include extreme flexibility in using facilitator-based delegation of complex goals, triggers, and data management requests; agent-based provision of multimodal user interfaces; and built-in support for including the user as a privileged member of the agent community. This article explains the structure and elements of agent-based systems constructed using OAA. The characteristics and use of each major component of OAA infrastructure are described, including the agent library, the Interagent Communication Language, capabilities declarations, service requests, facilitation, management of data repositories, and autonomous monitoring using triggers. To provide technical context, we describe the motivations for OAA's design, and situate its features within the realm ofalternative software paradigms. A summary is given of OAA-based systems built to date, and brief descriptions are given of several of these.